This invention relates to time division busses and more particularly to such busses having multi-point, bidirectional capability and operable for accepting circuit boards physically located at many points on a long bus.
High density time division busses suffer from many problems, one of which is that impedance discontinuities cause signal reflections to occur along the bus. These reflections, in turn, affect the settling time and noise margins of the bus and thus reduce the time "window" in which valid signals may be received at a receiving board.
Impedance discontinuities result from many factors, some controllable and some uncontrollable. One such uncontrollable discontinuity results from the removal (or non-placement) of a circuit board from a particular physical position along the bus.
This problem is compounded for bidirectional busses where data is flowing in two directions along the bus. In such situations the window must be made even more narrow. Typically, this problem is overcome by establishing two busses for interconnecting the circuit boards, one bus for signals passing from all circuit boards and a separate bus for signals passing to all circuit boards. A common control circuit would receive and retransmit all signals according to a reference clock.
In addition to signal propagation delays along the bus, account must be taken of the transmission response time delays of each individual board. In operation, a given board would receive a clock signal telling it to transmit whatever data is currently stored in its buffer. This signal must first be processed and then must turn on the necessary bus drivers in order to deliver the data to the bus. Accordingly, some period of time, called the transmission delay, occurs before valid signals are placed on the bus. This problem is compounded when a clock delay between boards is considered. These delays further reduce the window for valid data.
Thus, a need exists in the art for a bidirectional bus arrangement which allows for the spanning of long distances while still containing a transmission window wide enough for transmission in either direction between boards located at the extreme ends under all conditions.